The present invention relates to a sensor for detecting bleed out in the direct chill casting of metals.
Direct chill casting of metal is a widely used technique in the production of metals and alloys. Two general classes of direct chill casting techniques are used, these being vertical casting and horizontal casting.
Vertical direct chill casting is widely practised in the aluminium industry. This process is a semi-continuous process. In this process, a mould having water cooled side walls and an open top and bottom is provided. A dummy block is used to close the bottom of the mould during start up. Once the dummy block has been positioned in the mould, molten metal is poured into the mould. As the walls of the mould are water cooled, the molten metal that contacts the side walls is cooled very quickly and a skin of frozen or solidified metal forms adjacent the side walls. Due to contraction upon cooling, this skin pulls away from the side walls of the mould.
Once the metal level in the mould reaches the desired level the dummy block is lowered at a preset rate and metal flow to the mould is controlled to match the descent rate and maintain a substantially constant metal level in the mould. As the solidified skin of metal in the casting exits the bottom of the mould, further cooling is provided by spraying water directly onto the casting. This causes rapid solidification within the casting. The casting process continues until the dummy block is lowered to the full extent of its travel or the casting reaches its desired length. When this point is reached, flow of molten metal to the mould is stopped. After allowing sufficient time for solidification, the cast object is removed, the dummy block replaced in the bottom of the mould and the next cast commences.
During the casting process, the solidified skin of metal on the casting surrounds a sump of liquid metal within the casting. The sump of liquid metal within the casting can extend a significant distance below the bottom of the mould. If the skin of solidified metal is breached, the liquid metal can flow out through the breach. This is known as bleed out. Bleed outs are potentially dangerous events and a real risk of a molten metal explosion exists if bleed outs occur.
When blocks or rolling ingot are being cast the moulds are supplied with metal on an individual basis. In other words each mould has its own metal flow controlling device that maintains a given level of liquid in the mould during the cast. When a bleed out occurs in this case the controller can be programmed to sense that the flow of metal into the mould is significantly above what is required. It therefore senses that something is wrong and aborts the cast. This is an indirect method of detecting a bleed out. A similar situation exists with T-bar casting as the number of moulds is small and they are fed individually.
Normal operating practice in the event of a bleed out is to plug the mould or otherwise stop the flow of molten metal to the mould in which the bleed out is taking place. However, the configuration of industrial vertical direct chill casting operations can make bleed out detection difficult. Commercial vertical direct chill casting operations may utilise a number of moulds fed from a common source, such as a flooded table. The dummy blocks associated with the moulds are normally connected to one or a few common lowering means. If a bleed out occurs in a mould in the middle of a large table it can be difficult to detect. If only a single mould was being fed with liquid metal, a bleed out would result in a rapid lowering of the metal level in the mould. This opens the possibility of monitoring metal level to detect bleed outs in such a system. However, when a flooded table is used to feed a large number of moulds (which, in practice, can be up to 90 or more) bleed out in one mould has a very small effect on metal level. Therefore, monitoring of metal level is not effective to detect bleed outs in such apparatus.
Other types of moulds used in direct chill casting known to the person skilled in the art include those used in rolling block and T-bar casting.
Currently, there are no direct methods of sensing when a bleed out has occurred. It is an object of the present invention to provide a bleed out detector or sensor for direct chill casting.